The present invention relates to heterojunction bipolar transistors including those connected in parallel and methods for fabricating the same. The invention also relates to a high-frequency transmitter or receiver having a heterojunction bipolar transistor as an amplifier.
As a high-power device for microwave band, there has been developed GaAs-based heterojunction bipolar transistors (hereinafter, referred to as HBTs). Generally, HBTs, which are high in thermal resistance, have a problem that when used as a high-power device, HBTs would involve high junction temperature. On this account, as shown in FIG. 22, a structure for improving heat radiation property has been proposed in Japanese Patent Laid-Open Publication HEI 8-279562. FIG. 22A shows a planar pattern of HBTs connected in parallel for high-power operation, and FIG. 22B shows a cross section taken along a line B—B of FIG. 22A. In this structure, a plurality of HBTs 90 each having a collector electrode 106, a base electrode 105 and an emitter electrode 104 are included on the surface side of a GaAs substrate 113, where via holes 110 are provided between adjacent HBTs 90 so as to be cut through the substrate from its top to rear surface side. Heat generated at a junction 127 on the top surface side of each HBT 90 is conducted from the emitter electrode 104 of the transistor to a metal body 99 within its adjacent via holes 110 via an air bridge 111, and further conducted from the metal body 99 to a plated heat sink (hereinafter, referred to as PHS) layer 112 provided on the substrate rear surface, thus being radiated.
However, this conventional structure has a first drawback that for implementation of even higher power output, electric resistance of the air bridge 111 is not negligible, with heat radiation effect insufficient, so that the junction temperature inside the transistor cannot be reduced sufficiently. In this conventional structure, there is a second drawback that because of limitations in reducing the emitter inductance, there may arise variations in high frequency characteristics or the gain in high frequency operation may decline.